Cover film stretching device

ABSTRACT

The invention relates to a packaging machine with which packaging cavities ( 3 ) are filled with a product to be packaged and are closed by a cover film ( 2 ). The packaging cavities ( 3 ) are displaced in an intermittent manner along the packaging machine, and the cover film is stretched to its exact positioning on the packaging cavities. The invention also relates to a method for sealing a cover film on packaging cavities.

The present invention relates to a packaging machine, by means of which packaging troughs are filled with packagable material and closed by a cover film, wherein the packaging troughs are moved intermittently along the packaging machine and the cover film is stretched for its exact positioning above the packaging troughs. The present invention further relates to a method of sealing a cover film onto packaging troughs.

Nowadays packaged goods, in particular foodstuffs, are being packaged increasingly often in plastic packs. These plastic packs consist of a so-called packaging trough, into which the packagable material is introduced, and a cover film, which is sealed onto the packaging trough and by which the packaging trough is closed. As these cover films are increasingly often imprinted with trade marks or product information, an exact positioning of the printed image of the cover film above the packaging trough has to be undertaken prior to sealing. Furthermore, for an attractive appearance of the packaging it is desirable for the cover film to be sealed in a tightly stretched manner onto the packaging trough. This is achieved in particular by the printed images of the cover film being slightly smaller than the cross section of the packaging troughs that is to be sealed, with the result that the cover film has to be stretched (oriented) prior to sealing.

At present, this stretching is effected by the forward draw of the packaging troughs in that unwinding of the cover film is stopped shortly before completion of the forward draw of the packaging troughs. This procedure does however have the drawback that the printed image of the cover film is sometimes not positioned precisely enough above the packaging troughs.

The underlying object of the present invention is therefore to provide a packaging machine, and a method of sealing the cover film onto packaging troughs, with which the printed images of the imprinted cover film may be positioned very precisely above the packaging troughs and sealed onto them.

The object is achieved according to the invention by a packaging machine, by means of which packaging troughs are filled with packagable material and closed by a cover film and in which the cover film is stretched for its exact positioning above the packaging troughs and which has a stretching device that is not based on the movement of the packaging troughs.

For the person skilled in the art it was extremely surprising and unexpected that by means of the packaging machine according to the invention the printed images of the cover film are successfully sealed extremely precisely onto the packaging troughs. The packaging machine according to the invention is easy and economical to manufacture and to operate.

In a preferred embodiment of the present invention, the stretching device has a clamping means, by which the cover film is fixed in such a way that it may be stretched. As a clamping means, any clamping means known to the person skilled in the art and by which fixing of the cover film may be effected rapidly, reversibly and without impairment of the cover film, is conceivable. An inflatable tube is mentioned here purely by way of example.

The stretching device moreover preferably has a shifting means, by which the clamping means may be shifted in such a way that the cover film stretches to the desired length. The shifting means is preferably infinitely adjustable.

In a particularly preferred embodiment, the packaging machine according to the invention has a photocell, by means of which print marks disposed in each case between two printed images are detected.

In a further preferred embodiment, the cover film is wound into a film roll, which in turn is supported on a roll holder. The cover film is preferably tensioned by means of a dancer roll.

The present invention further relates to a method of sealing a cover film onto packaging troughs that are moved intermittently along a packaging machine, whereby the cover film is stretched for its exact positioning above the packaging troughs and then sealed and whereby the stretching of the cover film is effected without the aid of the movement of the packaging troughs.

The method according to the invention has the advantage that the printed images on the cover film may be positioned very precisely above the packaging troughs. The method is easy and economical to implement.

Preferably, the cover film is fixed, in a particularly preferred manner sealed, onto at least one packaging trough and the cover film is unwound from a film roll by the intermittent forward draw of the packaging trough and, as soon as the forward draw of the packaging trough is complete, is fixed by a clamping means, stretched and then sealed.

In a particularly preferred embodiment, during the unwinding of the cover film the actual position of print marks is detected. The actual position thus determined is compared with a setpoint position and, based on the comparison, the measure of stretching (orienting) of the cover film is determined. In order to prevent over-stretching of the film, the measure of maximum stretching is limited by the film formulation.

There now follows a description of the invention with reference to FIGS. 1 and 2. This description is purely by way of example and does not limit the general spirit of the invention.

FIG. 1 shows a cover film stretching device having a photocell and having a stretching device with positional data.

FIG. 2 shows a cover film stretching device having a displaceable photocell.

FIG. 1 shows the packaging machine according to the invention (only partially illustrated), in particular the stretching device and sealing unit. The packaging troughs 1 are conveyed along the packaging machine in the conveying direction denoted by arrow 12. The person skilled in the art will appreciate that in the paper plane a plurality of packaging troughs may be disposed one behind the other. In the sealing chamber 14, the cover film 2 is sealed onto the packing troughs 1′ situated at that time in the sealing chamber. For this purpose, the lower sealing tool 15 is moved up and down, as illustrated by the double arrow, so that the cover film 2 is sealed under the influence of temperature and pressure onto the packaging troughs 1′. As soon as the sealing operation is complete, the packaging troughs are moved on. As the cover film is connected to the packaging troughs 1′, upon forward feed of the packaging troughs the cover film 2 is unwound from the film roll 9, which is disposed on the roll holder 10. During the unwinding operation, the actual position of the print marks 8, which are situated on the cover film and delimit in each case one printed image, is determined and in a central control unit the difference between the actual value and a defined setpoint value is determined. This difference is then the measure for the stretching length, which arises from the difference between actual value and setpoint value. On completion of the forward draw of the packaging troughs, the clamping means 5, which in the present case is an inflatable tube, is activated and fixes the cover film 2. The cover film is then stretched by the calculated measure (oriented) by the shifting means 6. For this purpose, the shifting means is displaced counter to the direction of travel of the cover film. The shifting means in the present case is driven by a stepping motor or servomotor. As the cover film is fixed in the stretching direction 4, it is oriented (stretched). As soon as the stretching of the cover film is complete, i.e. the cover film is positioned precisely above the respective packaging troughs, sealing is effected in the previously described manner. On completion of the sealing operation, the clamping means 5 is released and the shifting means 6 is returned to its starting position. The movement of the shifting means is symbolized by the double arrow.

When processing a new cover film, the method is modified as follows. First, the printed image of the upper film is placed as precisely as possible onto the lower film and/or the packaging troughs deep-drawn therein and then by means of a measurement the distance between the photocell 7 and the reference print mark 8′ is determined. This distance is the so-called setpoint value, which is communicated for example via a control panel to a central control unit. The machine is then switched on and the cover film 2 is sealed onto the packaging trough 1′. Upon each forward draw of the lower film having the packaging troughs, the cover film sealed onto these packaging troughs is unwound and the actual position of the print marks is determined by the photocell 7. The difference between the setpoint position and the actual position is the measure of the respective stretching length. Should the required stretching length exceed the maximum stretching length permissible for the material, the stretching of the cover film is limited to this maximum stretching length. For the stretching operation, the upper film on completion of the forward draw is fixed by the clamping means 5 and the shifting means 6 is displaced counter to the direction of travel of the film in accordance with the defined stretching length. The determined setpoint value is stored in the formulation of the film and/or of the product associated with this film in the machine.

A change of cover film merely involves inserting the film into the packaging machine and downloading the appropriate formulation and hence the setpoint value from a central control unit. After start-up of the machine, the printed image is then automatically positioned by the system extremely precisely above the lower film and/or the packaging troughs therein by virtue of determination of the print mark actual position and subsequent stretching. The system also limits the maximum stretching length per stretching operation to prevent tearing of the cover film. This maximum stretching length is likewise included in the stored formulation.

FIG. 2 shows a cover film stretching device according to FIG. 1, except that in the present case the photocell is supported displaceably, this being illustrated by the double arrow. The stretching device according to FIG. 2 moreover has, instead of a shifting means with positional data, a shifting means with a position cut-out. Otherwise, the description pertaining to FIG. 1 applies. As already mentioned, the photocell is displaceable in and counter to the direction of travel of the film. By means of the, in the present case, manual displacement of the photocell the setpoint position of the print mark is adjusted for the respective type of film. The photocell first of all registers whether during the forward draw of the film a print mark has run past the photocell. On completion of the forward draw, the stretching device 4 stretches the cover film 2 until the photocell 7 once again detects the print mark that has run past. In the present case, the stretching device 4 is driven by a cylinder (not shown), to which no positional data may be supplied. Upon detection of the print mark, the stretching operation is terminated and the film is fixed by sealing onto the packaging troughs. In the event of non-detection of the print mark, the full available stroke of the cylinder is executed.

This embodiment of the present invention is suitable in particular for drives of the stretching machine 4 without positional data, such as e.g. cylinders.

All of the explanations given with regard to the apparatuses according to the invention apply equally to the method according to the invention and vice versa.

LIST OF REFERENCE NUMERALS

-   1, 1′ packaging trough -   2 cover film -   3 packaging trough -   4 stretching device -   5 clamping means -   6 shifting means -   7 photocell -   8, 8′ print marks, reference print mark -   9 film roll -   10 roll holder -   11 dancer roll -   12 conveying direction -   13 rollers -   14 sealing chamber -   15 lower sealing tool 

1. Packaging machine, by means of which packaging troughs (1) are filled with packagable material and closed by a cover film (2), wherein the packaging troughs (1) are moved intermittently along the packaging machine and the cover film (2) is stretched for its exact positioning above the packaging troughs (1) counter to its direction of travel by means of a stretching device (4) that is not based on the movement of the packaging troughs (1), characterized in that the stretching device (4) has a clamping means (5) for the cover film (2) as well as a shifting means (6).
 2. (canceled)
 3. Packaging machine according to claim 1, characterized in that it has a photocell (7), which detects print marks (8).
 4. Packaging machine according to claim 1, characterized in that the cover film is supported as a film roll (9) on a roll holder (10).
 5. Packaging machine according to claim 1, characterized in that the cover film (2) may be tensioned by means of a dancer roll (11).
 6. Method of sealing a cover film (2) on packaging troughs (1) that are moved intermittently along a packaging machine and whereby the cover film (2) is stretched for its exact positioning above the packaging troughs (1) without the aid of the movement of the packaging troughs (1) and then sealed, characterized in that the cover film (2) is fixed by a clamping means (5) and stretched counter to the direction of travel by a shifting means (6).
 7. Method according to claim 5, characterized in that the cover film (2) is fixed to at least one packaging trough (1) the cover film (2) is unwound from a film roll (9) by the movement of the packaging troughs (1).
 8. Method according to claim 6, characterized in that during the unwinding of the cover film (2) the actual positioning of print marks (8) is detected.
 9. Method according to claim 8, characterized in that the measure of the stretching is determined on the basis of the difference between actual positioning and setpoint positioning.
 10. Method according to claim 9, characterized in that the measure of the stretching is limited on the basis of the film formulation. 